DONETSK, Ukraine — One civilian was killed in shelling in the Ukrainian port city of Mariupol, and large explosions were heard near the airport in Donetsk early Sunday, raising fears that a cease-fire signed two days ago is on the verge of collapse.

Blasts from the area near the airport were powerful enough to be heard in downtown Donetsk, the main rebel-held city in eastern Ukraine.

A spokesperson for Ukraine's national security council, Volodymyr Polyovyi, said at a briefing in Kiev that rebels appeared to have tried to attack the airport, which has been under the control of government troops since May, and has come under unremitting attacks from pro-Russia separatist rebels since then.

Later on Sunday, at least two houses blazed in the rural village of Spartak, which lies just north of Donetsk and adjacent to the airport, after they were hit by fire. A man whose house was struck by a shell said rebels had fired from a spot nearby, which appeared to have provoked a retaliatory attack from Ukrainian government troops. This pattern has been regularly observed in the nearly five-month-long military confrontation.

A group of rebel fighters in the village danced and drank Sunday morning in celebration after what they said was a successful assault on a Ukrainian military encampment in the vicinity. One said their group had captured eight government troops, although none of these captives could be seen.

The fighter, who provided only the nom de guerre Khokhol, freely acknowledged that the cease-fire was not being respected by either side.

"There was mortar shelling around 20 minutes ago here in Spartak," he said. "There is no cease-fire for anyone."

The cease-fire had appeared to be holding for much of the day on Saturday, but shelling started late at night. A rebel statement said Ukrainian forces violated the cease-fire by firing on their positions in six locations on Saturday, including near the Donetsk airport. The statement said several rebels were killed.

Shelling also occurred overnight on the outskirts of the port city of Mariupol, where Ukrainian troops retain defensive lines against the rebels.

The city council there said one civilian was killed and a serviceman was wounded. A shell also destroyed a nearby gas station, and the volunteer Azov Battalion said on Facebook that their positions were hit by Grad rockets, but did not give details.

Mariupol is located on the coast of the Sea of Azov, 115 kilometers (70 miles) south of Donetsk. Rebels recently opened a new front on the coast, leading to fears that they were trying to secure a land corridor between Russia and the Crimean Peninsula, which Russia annexed in March.

Amnesty International on Sunday condemned all those engaged in the grinding conflict. "All sides in this conflict have shown disregard for civilian lives, and are blatantly violating their international obligations," Amnesty International secretary general Salil Shetty said in a statement.

Echoing similar allegations by the Ukrainian government and NATO, Amnesty International said that it has evidence that Moscow is fueling the conflict through direct support for separatist fighters. In making its case, the group presented satellite images appearing to show Russian weaponry being brought into Ukraine.

"These satellite images, coupled with reports of Russian troops captured inside Ukraine and eyewitness accounts of Russian troops and military vehicles rolling across the border, leave no doubt that this is now an international armed conflict," said Shetty, who is set to visit Kiev and Moscow in the coming days.

Amnesty also said that the Ukrainian government has subject residential areas to heavy and indiscriminate shelling.

The group added that both pro-government and separatist militia groups had abducted and beaten people suspected of aiding their opponents.

Ukraine, Russia, the pro-Russian rebels and the Organization for Security and Cooperation in Europe signed the cease-fire deal in the Belarusian capital of Minsk on Friday in an effort to end more than four months of bloodshed. The negotiators also agreed on the withdrawal of all heavy weaponry, the release of all prisoners and the delivery of humanitarian aid to devastated cities in eastern Ukraine.

The 12-point agreement, published Sunday by the OSCE, also obliges Kiev to give greater powers to the separatist Donetsk and Luhansk regions, and calls for local elections to be held in those Russian-speaking regions.

Western leaders voiced skepticism over Russia's commitment to the deal. A previous 10-day cease-fire, which each side repeatedly accused the other of violating, yielded few results at the negotiating table.

The U.S. and the European Union have prepared to enforce tougher sanctions on Russia; however, the Russian Ministry of Foreign Affairs reportedly said that "there will undoubtedly be a reaction from our side" to any new EU sanctions.

Ukrainian President Petro Poroshenko's office on Saturday said he and Russian President Vladimir Putin had discussed steps "for giving the cease-fire a stable character" in a telephone conversation. But, it said, both leaders assessed the cease-fire as having been "fulfilled as a whole."

Fighting between pro-Russian rebels and Ukrainian government troops has ravaged the already teetering Ukrainian economy, claimed at least 2,600 civilian lives and left hundreds of thousands homeless, according to United Nations estimates.

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